The present invention relates to wireless communication systems of the type using whitespaces, and more particularly to methodology for allocating a frequency to be used.
With remarkable advances in information technology (IT) society in recent years, wireless communication other than wired communication is widely used as a communication method for use in many IT devices and services. This leads to a continuous increase in demand for radio spectrum which is a limited resource, resulting in creation of a big problem—that is, use-up or “depletion” of assignable frequencies in countries all around the world. In general, radio spectrum is license-managed by a country to make sure that only a licensed person is granted to use spectrum under strict control in a specific geometric location at specific time. However, in order to meet ever-increasing frequency demands, it is desired to contrive a new spectrum-using method that breaks through the concepts of conventional utilization methods.
Consequently in recent years, there has been studied as such new spectrum use method for solving the spectrum depletion problem a method of utilizing certain frequency bandwidths (whitespaces) that remain unused in terms of space and time in spite of the fact that these have already been allocated. For example, research is conducted to develop the so-called cognitive wireless communications system which sufficiently avoids the influence on spectrum usage of incumbent systems of licensed users (referred to hereinafter as primary users) and, simultaneously, permits unlicensed users (referred to as secondary users) to flexibly utilize radio waves of whitespaces. Teachings as to this system are found, for example, in H. Fujii, “Cognitive Radio: Core Technology of White Space Utilization for Avoiding Waste of Radio— wave Usage,” [online], RIC TELECOM, (Searched on Jun. 9, 2011), Internet (URL: http://businessnetwork.jp/tabid/65/artid/110/page/1/Default.aspx).
In one exemplary wireless communication system that uses whitespaces in a way compliant with the IEEE 802.22 standard, each wireless station provides access to a database on IP network to thereby acquire both maximal transmission-capable electrical power and a list of transmittable frequencies based on its own position information. The transmittable frequency list is centrally managed, with opportunistic updating, by a spectrum manager (SM) in a base station (BS) which is connected to customer premises equipment (CPE), such as per-house installed substations, mobile/cellular telephone handsets and others. Based on this transmittable frequency channel list, the BS specifies a frequency utilizable for bidirectional communications between BS and CPE and selects it to be a use frequency.
Respective wireless stations (i.e., BS and CPE; the same goes for those indicated below) have spectrum sensing functions. When each wireless station detects that the use frequency which was selected by the spectrum sensing is currently used by an incumbent system (i.e., the primary system/primary user's system), it notifies the SM of the information showing this fact. In responding thereto, SM excludes this frequency from the transmittable frequency list. In this way, the wireless communication system using whitespace performs dynamic spectrum access based on such momentarily updated information, thereby establishing communications between secondary users while at the same time avoiding the influence on the primary user's spectrum usage.
Incidentally, the IEEE 802.22 standard defines only the time division duplex (TDD) as duplex communication scheme. Although BS and CPE use the same frequency in data transmission events and perform communications at different transmission timings to thereby realize two-way communications. However, a duplex communication method relying on the TDD only uses the same frequency for two-way communications. Thus, it is hard to say that the TDD is always an effective communication method for the whitespace-use wireless communication system, which will possibly be encountered with cases where the transmittable frequency differs per wireless station and where maximum transmittable electric power is different. For example, in a case where a wireless station at one end of a communication channel is able to perform high-power transmission at a frequency whereas its associated wireless station at the other end of channel can merely perform low-power transmission at such the frequency, two-way communication quality becomes asymmetrical. Thus, allocating such frequency makes it difficult to perform communications effectively.
Alternatively, in case one wireless station is able to transmit data at a frequency whereas the other wireless station is unable to perform transmission at this frequency, communication therebetween becomes a one-way link which can merely perform data transmission/reception in only one direction, i.e., simplex communication. Obviously, such frequency is unusable in TDD of IEEE 802.22.